Microphone having resonant compensating means



Oct. 18, 1955 sc 2,720,934

MICROPHONE HAVING RESONAN'I COMPENSATING MEANS Filed Jan. 27, 1953 IN VEN TOR $5 .5

GERRIT SCHENKEL BY %W% AGENT United States Patent MICROPHONE HAVING RESONANT COMPENSATING MEANS Geert Schenkel, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application January 27, 1953, Serial No. 333,497

Claims priority, application Netherlands March 20, 1952 4 Claims. (Cl. 181-5) The invention relates to a microphone comprising a diaphragm and at least one pipe, with which the diaphragm is coupled acoustically and which is open at the other end, where it is provided with means to compensate the drop of the frequency characteristic curve in the range of the higher frequencies, for example, of more than 2000 C./ S.

A conventional form of the said compensating means consists of a pipe closed at one end, for example, shaped in the form of a ball having a cylindrical bore, in which the open end of the pipe opens and in which, during the operation of the microphone, the air may start resonating in the frequency determined by the length of the pipe. The effect of the pipe on the frequency characteristic curve of the microphone becomes manifest in the increased sensitivity of the microphone in a more or less extensive frequency range, which lies substantially symmetrically about the resonance frequency of the pipe. Since, in the absence of the said compensating means the frequency characteristic curve mostly exhibits a continuous drop in sensitivity, if the frequency exceeds a definite value, the so-called decline frequency, for example, in excess of 2000 0/8., it will be obvious that in order to render the frequency characteristic linear, the resonance frequency should exceed the decline frequency.

The invention has for its object to provide means which permit of acting upon the frequency characteristic curve at different frequencies to be chosen independently of one another, in order to obtain as accurately as possible the desired linearity of the frequency characteristic even at the higher frequencies.

The microphone according to the invention is characterised in that the said compensating means are constituted by at least one tube, which is open at both ends and in which the said pipe opens out, its shape being such that its resonance frequency is followed up by the frequency range in which the frequency characteristic exhibits a decline of a few decibels, if the said means are not used. The microphone according to the invention is furthermore characterised in that, moreover, each of the two ends of the pipe opens out in a tube of larger diameter and smaller length, which permits of compensating the decline of the frequency characteristic even at still higher frequencies.

It is known that in the case of resonance of the air in a tube, open at both ends, the distribution of the pressure is symmetrical, the maximum pressure being at the center of the tube itself. It is therefore advantageous to construct the said means in a manner such that the open ice end of the pipe is located in the center of the tube surrounding it. In a particular embodiment the pipe and tubes are arranged coaxially.

It is furthermore advisable to have the pipe support the surrounding tube, which in turn serves as a support for other tubes.

In order that the invention may be readily carried into effect, it will now be described in detail with reference to the accompanying drawing, which shows one embodiment.

Fig. 1 is in part a side view and partly a cross sectional view of the microphone and Fig. 2 is a plan view of the means used in accordance with the invention.

Fig. 3 shows the effect of the said means on the frequency characteristic curve of the microphone.

According to Fig. 1 the microphone 1 is provided with a pipe 2, which is coupled acoustically at one end 3 in known manner with a diaphragm (not shown for the sake of simplicity) and the other end 4 is open and opens out in the center of a tube 5. This tube is also open at both ends and is supported and secured at three areas through connecting pieces 6 by the pipe 2. The ends of the tube 5 furthermore open out each in the centers of tubes 7 of smaller length and larger diameter, which are supported and secured by tube 5 through connecting pieces 8. It is furthermore evident from Figs. 1 and 2 that the pipe and the tubes are coaxial with one another.

According to Fig. 3 curve a corresponds to the case in which the said compensating means are not used, whereas curves b and c indicate the compensation of the decline of the characteristic curve in excess of the decline frequency, which is about 2000 0/5. in this case, if the tube 5, or tube 5 in conjunction with tubes 7 are used.

What I claim is:

1. A microphone comprising a diaphragm, at least one pipe coupled acoustically with the diaphragm at one end and open at the other end, and means coupled to the open end of the pipe for compensating for the decline of the frequency characteristic of the microphone at frequencies in excess of 2000 C. P. 8., said compensating means comprising at least one tube open at both ends surrounding the open end of the pipe and having a resonance frequency in excess of 2000 C. P. 5., said compensating means further comprising second and third tubes each of larger diameter and smaller length than said first tube and each surrounding an opposite open end of said first tube.

2. A microphone as claimed in claim 1 in which the open end of the pipe is located at the center of the first tube, and the open ends of the first tube are located at the center of the second and third tubes, respectively.

3. A microphone as claimed in claim 2 in which the three tubes and the pipe are coaxial with respect to one another.

4. A microphone as claimed in claim 1, in which the pipe supports the first tube, and the first tube supports the second and third tubes.

References Cited in the file of this patent UNITED STATES PATENTS 

